Yes the auction example is dated 1970; and note that the cane does have an integral "s" but it is not easily seen until the images are zoomed in several times.
See
this section of Richard More's site on Strathearn weights.
Richard's research tells us that date canes were used in some weights in all years from 1967 to 1980. As far as I know, there was no specific reason for some examples of a design to have a date cane and not others.
Richard points out that it appears that no date cane was used for 1965/6, but some weights had an "S" cane. Importantly, Richard also says, in his initial explanatory text that "nothing is etched in stone" and "all ... is subject to change ... as ... research produces new information".
An example of ongoing research is that it is now known that one of the "S" canes (see the first three examples in Richard's page that I linked to above) was actually made much earlier than the start of Strathearn Glass! The cane has been found in several weights for which uv analysis together with regular visual features, strongly suggest it was first made as an "S" for Salvador Ysart (who died in 1955).